THE ORGANIZER. Can’t believe that’s what it’s called. You can even boil “organizer cells” from another species, implant them, and they will organize ectoderm into neural tubes and skin, and organize mesoderm into notochord which becomes our spine. [the dorsal organizer creates the notochord)
Induction of embryonic primordia by implantation of organizers from a different species. 1923
Spemann and Mangold’s Discovery of the Organizer, 1923.
This I did not expect to find. I knew there’d be SOMETHING from the mesoderm (as the inbetween) that influenced the outer and/or inner layers… but this mystical “organizer” thing really is something to find.